In the implantable medical device field, a medical device, configured to perform a desired medical function, is implanted in the living tissue of a patient so that a desired function may be carried out as needed for the benefit of the patient. Numerous examples of implantable medical devices are known in the art, ranging from implantable pacemakers, cochlear stimulators, muscle stimulators, glucose sensors, and the like.
Some implantable medical devices are configured to perform the sensing function, i.e., to sense a particular parameter, e.g., the amount of a specified substance in the blood or tissue of the patient, and to generate an electrical signal indicative of the quantity or concentration level of the substance sensed. Such electrical signal is then coupled to a suitable controller, which may or may not be implantable, and the controller responds to the sensed information in a way to enable the medical device to perform its intended function, e.g., to display and/or record the measurement of the sensed substance. An example of an implantable medical device that performs the sensing function is shown, e.g., in U.S. Pat. No. 4,671,288.
As medical devices have become more useful and numerous in recent years, there is a continual need to provide very low power sensors that may be connected to, or incorporated within, such devices so that the desired function of the device can be carried out without the expenditure of large amounts of power (which power, for an implanted device, is usually limited.)
It is known in the art to inductively couple a high frequency ac signal into an implanted medical device to provide operating power for the circuits of the device. Once received within the implanted device, a rectifier circuit, typically a simple full-wave or half-wave rectifier circuit realized with semiconductor diodes, is used to provide the rectifying function. Unfortunately, when this is done, a significant signal loss occurs across the semiconductor diodes, i.e., about 0.7 volts for silicon, which signal loss represents lost power, and for low level input signals of only a volt or two represents a significant decrease in the efficiency of the rectifier.
For the extremely low power implantable devices and sensors that have been developed in recent years, low operating voltages, e.g., 2-3 volts, are preferable in order to keep overall power consumption low. Unfortunately, with such low operating voltages are used, a diode voltage drop of 0.7 volts represents a significant percentage of the overall voltage, thus resulting in a highly inefficient voltage rectification or conversion process. An inefficient voltage conversion, in turn, translates directly to increased input power, which increased input power defeats the overall design goal of the low power device. What is needed, therefore, is a low power rectifier circuit that efficiently converts a low amplitude alternating input signal to a low output operating voltage.
Further, it is not always possible to fabricate diode-type bridge rectifiers on CMOS or bipolar chips using conventional processing technology. It is particularly difficult to make a good connection with the non-substrate positive rail or positive supply of the chip. There is thus a need in the art for a low power rectifier circuit that generally avoids the use of problematic diodes.
Rather than diodes, switches may be used within a rectifier circuit. Such switches can be configured to exhibit an extremely low turn on voltage, e.g., on the order of 50 mV. Disadvantageously, before such switching circuits can operate, there must be an operating potential already available (supply voltage) that can bias (provide operating power to) the switches for their desired operation. In many implantable sensor applications, an operating potential will not exist until such time as the rectifier circuit rectifies the incoming power signal. Thus, rectification cannot occur until an operating potential is present, and an operating potential cannot exist until rectification occurs--a true stalemate. It is thus evident that critical improvements are needed in the rectification circuits used within low power implantable devices, such as implantable sensors, that are powered by an incoming ac or pulsed signal.